1. Technical Field
This invention is directed to a method and device for measuring flow rate of gas; in particular, a device and method for measuring gas flow in a pipe using a non-intrusive clamp-on flowmeter.
2. Description of Related Art
Transit-time ultrasonic Gas flowmeters are well known. FIG. 1 shows a conventional gas flowmeter wherein transducers are introduced directly into the gas via holes in the pipe wall. Since the pressure within the pipe is usually quite high, typically from several atmospheres to many hundreds of atmospheres, it is necessary to use a pressure seal around the transducers to prevent the escape of gas. Since the transducer itself must also withstand the pressure it must necessarily be rigid and of high density. Because sonic impedance is directly related to material density, i.e., Zs=material density times its sonic propagation velocity, the transducer/pipe interface presents a high sonic impedance, which is a mismatch to the low sonic impedance of gas. Accordingly, the efficiency of introducing sonic energy into gas is low. To overcome this inefficiency it is necessary to generate a substantial impulse at the emission surface of the transmit transducer 100. This results in substantial energy in the transducer body that would preferentially enter the pipe wall of pipe 120 to which it is attached, via the pressure seal, since the high impedance of the transducer matches the impedance of the pipe wall.
Once this energy enters the pipe wall it will pass directly to the receive transducer 110 through the pipe wall. As compared to the gas signal amplitude received at the receive transducer 110, the amplitude of the transmit energy is much higher, and the residence time in the pipe of the transmit energy is lengthy, as it reverberates between pipe wall discontinuities. Thus the much smaller gas receive signal amplitude is obscured. This makes detection of the effect of flow on the transit-time of the received gas signal difficult or impossible due to low signal to noise ratio.
In one solution, the inserted gas transducer is equipped with an internal means of dissipating the sonic energy not transmitted into the gas before it reaches the location of the transducer to pipe wall pressure seal. A combination of metallic and viscous elastomeric elements can be used to dissipate the sonic energy. The signal to noise ratio of hundreds to one, or even greater, can be achieved.
Generally, the operation of the conventional transit-time flowmeter includes a flow computer which by digitizing the received signal computes flow from the measured difference in upstream versus downstream transit-time, and the absolute measured transit-time, using well-known algorithms.
With the conventional transit-time flowmeter, a problem develops at high flow rates due to the fixed positions of the insert transducers in the pipe wall. The problem arises when the searchlight like xe2x80x9cnarrowxe2x80x9d beam width is generated and injected into the flow path of the gas. Since the flow rate of gas can reach a high percentage of the propagation velocity of the gas itself, it can be appreciated that under these conditions, xe2x80x9cblowingxe2x80x9d of the sonic beam downstream from both the upstream and downstream transducers can result. FIG. 2 illustrates such phenomenon. The normal beam transmitted from the transmit transducer 100 is xe2x80x9cblownxe2x80x9d downstream by the gas flow and the intended signal is not received by the receive transducer 110. Thus, there is no position to place either transducer that will avoid the possibility of having the sonic beam miss either or both, except perhaps to mount them axially in-line. This results however in the upstream transducer affecting the flow profile and even altering the apparent velocity of the sonic beam in the path between transducers.
It is well known that clamp-on liquid flowmeters produce highly accurate measurements. One important difference in the circumstances that affect gas versus liquid flowmetering is the very low sonic impedance of the gasxe2x80x94as much as 100 times or more lower than the average liquid case. This results in a substantial reduction of the signal amplitude of the sonic energy that enters the gas medium from the pipe wall. Another difference is the significantly lower sonic propagation velocity of gas as compared to a typical liquidxe2x80x94about one-fifth of typical liquid velocity. This lower propagation velocity also results in a much larger refraction angle by which sonic energy emerges from the pipe wall to enter the gas, relative to that for liquid. FIG. 3 illustrates the differences in refraction angles for gas and liquids. The gas sonic velocity is about 12,000 inches/second, while the liquid sonic velocity is about 60,000 inches/second, whereas the pipe wall sonic velocity is about 115,000 inches/second. This results in an angle from the normal of about 6 degrees for gas, versus as much as 26 degrees for liquid. This means that the sensitivity of flow detection is much smaller for gas than for liquid, in the ratio of the sine of these angles. However, the considerably higher flow rates common for gas as compared to liquid conveniently compensate for this fact.
In the conventional intrusive ultrasonic gas flowmeter, the design of the transducers prevents the entry of sonic energy into the pipe wall. But in the case of a clamp-on gas meter, it is essential to introduce a substantial sonic signal into the pipe wall, since it is the actual source of the sonic energy that enters the gas medium. Accordingly, a solution is to dissipate the remaining pipe wall sonic energy, the source of the gas signal, before the gas signal completes its path to the receive transducer through the gas. Fortunately, the velocity through the gas is very slow compared to the very high velocity of the sonic energy in the pipe wall. The typical ratio of these velocities is about 10:1.
If the transducers are clamped to an installed pipe which is very long between pipe wall discontinuities, such as flanges, then the transmit signal in the pipe wall will xe2x80x9cleave the vicinityxe2x80x9d and be absorbed, in the main, before it reflects from these flanges and obscure the very small gas signal. FIG. 4A shows reflection characteristics of noise amplitude versus distance of a direct wave in a spool. FIG. 4B shows reflection characteristics of undamped and a damped spool. FIG. 4C shows reflection characteristics of noise amplitude versus time of a direct wave. FIG. 4D shows reflection characteristics of noise amplitude versus time of an undamped and a damped spool. It can be seen that if the section of pipe is small, then an echo chamber is formed, and the original transmit sonic energy will be present at the location of the receive transducer when the gas signal arrives. Accordingly, for field mounted clamp-on gas flowmeters, it is recommended that the section of pipe on which the transducers are installed be long between discontinuities. The actual length required to obtain a designated level of performance, limited by signal to noise ratio, is shorter for higher pressure gas, which higher pressure serves to increase the amplitude of the gas signal.
But, since proving the calibration of a gas meter in the field is difficult at best, and is usually a practical impossibility, it is desired to fabricate a gas meter in the form of a spool. This permits it to be taken to a laboratory for calibration. But if it is necessary to make the spool long, as noted above, then this length makes it impractical to fabricate, calibrate, transport and install. Accordingly, it is desired to make the gas spool short. To accomplish this it is essential that a means be developed to dissipate the transmit sonic energy well before the gas signal arrives at the receive transducer.
A method and a device are provided for enabling the measurement of the flow rate of gas by use of a non-intrusive clamp-on transit-time ultrasonic flowmeter. The device and method utilize a wide beam transit-time transducer, in which its frequency matches the thickness mode and longitudinal mode resonance frequency of the pipe, and the phase velocity of the transducers matches the propagation velocity of a transverse mode wave in the direction of the axis of the pipe. These conditions assure that the sonic beam retains its shape as the emergence point from the pipe wall is moved by the higher gas flow rates, relative to the flow rate of the sonic pulse through the gas medium.
Disclosed is a means of dissipating the very high transmit sonic energy in the pipe wall so that it is smaller than the relatively low received gas signal limited by the inefficiency of introducing sonic energy from the high sonic impedance of the metallic pipe wall into the low sonic impedance of the gas. Such energy is naturally dissipated if the clamp-on transducer is mounted onto an installed gas pipe in which there is a large distance between the location of the transducers and the nearest pipe wall discontinuity, such as a flange or weld. However, if it is desired to manufacture a clamp-on gas spool, in which it is desired to limit the length of the spool, then the sonic energy will be xe2x80x9ctrappedxe2x80x9d between these discontinuities, and continue to reverberate past the transducer location and so be present and of substantial amplitude when the gas path received sonic wave arrives, and so obscure its detection.
Also disclosed is a gas flowmeter comprising a transmit transducer for injecting sonic energy into gas; a receive transducer for receiving the sonic energy; and a spool of pipe having a metallic pipe wall and a liner having a lower sonic impedance than the sonic impedance of the metallic pipe wall, wherein the transmit and receive transducers are mounted on the spool. Accordingly, the liner acts to absorb or dissipate the high sonic energy to dramatically improve the signal to noise ratio of the device.
Preferably, the pipe includes inner and outer pipe walls and the liner is formed on the inner pipe wall surface, but the liner may also be formed on the outside of the pipe wall, or both inner and outside surfaces. The liner may be bonded to the wall of the pipe by melting liner material onto the wall of the pipe declining the temperature at a given rate. The spool of pipe includes flanges and a reducer.
In another aspect of the invention, the liner is machined to a thickness to match the sonic wave guide properties of the pipe. The receive transducer is spaced at a distance for the transmit transducer such that a sonic beam transmitting from the transmit transducer arrives at the receive transducer at the same time via the pipe wall and via refraction from an opposing pipe wall of the pipe. In a preferred embodiment of the invention, the transmit transducer and the receive transducer is mounted along the same side of the pipe.
According to an alternative embodiment according to the present invention, a gas flowmeter comprises a spool of plastic pipe for carrying gas therethrough and a plastic transmit transducer and a plastic receive transducer mounted on the spool for injecting sonic energy into the path of gas flow, wherein the sonic energy is received by the receive transducer for determining velocity of gas flow.
Also disclosed is a method for measuring gas flow comprising the steps of injecting from a transmit transducer mounted on a metallic spool of pipe sonic energy into gas flowing through the spool of pipe; and receiving the sonic energy at a receive transducer through a liner bonded to the pipe wall of the spool of pipe, the liner having a lower sonic impedance than the sonic impedance of the metallic pipe wall. The liner is preferably made from a plastic. The pipe includes inner and outer pipe walls and the liner is formed on one of the inner or outer pipe walls.
In another aspect of the invention, the liner is bonded to the wall of the pipe by melting liner material onto the wall of the pipe and continually declining the temperature at a given rate.
In a preferred embodiment of the invention, the transmit transducer is spaced from the receive transducer at a distance such that a sonic beam transmitting from the transmit transducer arrives at the receive transducer at the same time via the pipe wall and via refraction from an opposing pipe wall of the pipe. The transmit transducer and the receive transducer are mounted along the same side of the pipe.
According to another preferred embodiment of the present invention, a gas flowmeter device for measuring gas flow by injecting and receiving sonic energy through the path of gas flow, comprises a spool of pipe for passing gas flow therethrough; and interface means disposed in the pipewall of the spool for sonic impedance matching materials in the path of the sonic energy. The interface means includes a liner having a lower sonic impedance than the sonic impedance of the pipewall and the spool of pipe is metallic. Alternatively, the interface means includes a plastic transmit transducer for injecting the sonic energy and a plastic receive transducer for receiving the sonic energy, and the spool of pipe is also plastic.